ASSOCIATIONS 



595 



host form elongate vermiform bodies called nematogens. These reproduce 

 asexually for many generations, but in the adult cephalopod new forms 

 called rhombogens appear. The rhombogens eventually give rise to free- 

 swimming infusoriform larvae which escape from the host, and serve to 

 infect new and possibly intermediate hosts. Dicyemids apparently are harm- 

 less parasites in cephalopods (32, 48, 64). 



Orthonectids are internal parasites of various flatworms, nemertines, 

 annelids, ophiuroids and a bivalve mollusc. The asexual form is an amoe- 

 boid syncytium which spreads in the tissues and body cavities of the host 

 and often causes considerable damage. Rhopalura ophiocomae, for example, 

 destroys the gonads of the brittle stars which it infects. Asexual reproduc- 

 tion is accomplished by fragmentation, but eventually sexual individuals 



Fig. 14.9. Piercing Stylets and Haemophilic Glands in the Head of the 

 Parasitic Syllid Ichthyotomus sanguinarius. (From Eisig (23).) 



are formed which escape from the host and swim about freely in the sea. 

 Fertilization takes place and the free-swimming larvae which result pene- 

 trate new hosts (48). 



Annelids. Only a few polychaetes have turned to parasitism, and these 

 do not show any profound alteration in structure. A syllid Ichthyotomus 

 sanguinarius, extensively investigated by Eisig (23), provides some interest- 

 ing details. This animal attaches itself to the fins of eels (Myrus, Conger) 

 by a pair of scissor-shaped stylets, which are opened out to hold the para- 

 site in position. The worm feeds on the blood which escapes from the 

 wound. Ichthyotomus still bears most of the characteristics of a free- 

 living syllid, but the cephalic appendages have disappeared and the 

 specialized stylets appear to be derived from the buccal teeth of carnivorous 

 forms. In the head there are two pairs of large haemophilic glands, which 

 secrete an anti-coagulant, and these have their homologues in the cutaneous 

 glands of non-parasitic syllids (Fig. 14.9). 



